全新java初学者实践教程10(java SE5.0版)
C &FN#B jdk5的集合类
QS%,7'EG wK ][qZ ] {V=vnL-- gpsrw>nw 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
B~4mk 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
~q5-9{ma [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
a(g$ d2H |'@V<^ GR [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
'iGMn_& W=M<
c@ [font="Times](1) Set
u4C1W|x 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
<JJkki h
bdEw=r? ([font="Times]2)[font="Times]List
z.{HD9TD 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
r5Wkc$ YBeZN98Nt FqL`Kt [font="Times] ([font="Times]3)[font="Times]Map
0A@-9w=u [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
yh4jRe?f =^ gvZ|] @V7;TJk [font="Times] ([font="Times]4)[font="Times]Queue
"&|lO| [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
!__D}k, [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
@gY'YA8m EqYz,%I% 0.3^ 实践:[font="Times] Set举例
a?l_-Fi [font="Times] import java.util.*;
!HbqbS22 [font="Times]public class SetExample {
*di&%&f [font="Times] public static void main(String[] args) {
.;cxhgU [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
<&*#famX [font="Times] set.add("one");
&boj$ k!g[ [font="Times] set.add("second");
i<0D
Z_rub [font="Times] set.add("3rd");
o<~-k,{5P [font="Times] set.add(new Integer(4));
m*OLoZVy [font="Times] set.add(new Float( 5.0F ));
]9dx3<2_I [font="Times] set.add("second");
<[esA9.]t [font="Times] set.add(new Integer(4));
m]Hb+Y=;h [font="Times] System.out.println(set);
Hs.6;|0% [font="Times] }}
r=xTs,xx ZKZl>dDuh [font="Times]List举例:
Bi$
0{V Z8 [font="Times] import java.util.*;
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@afE~ [font="Times]public class ListExample {
Dg1kbO=2 [font="Times] public static void main(String[] args) {
:Xh_$4~^Y [font="Times] List list = new ArrayList();
SxnIX/]J [font="Times] list.add("one");
#IH<HL)t%e [font="Times] list.add("second");
qZ `n Zi [font="Times] list.add("3rd");
YLD-SS[/> [font="Times] list.add(new Integer(4));
6yy|V~5 [font="Times] list.add(new Float( 5.0F ));
<=#lRZW[z [font="Times] list.add("second");
)R8%wk?2 [font="Times] list.add(new Integer(4));
A!Knp=Gw [font="Times] System.out.println(list);
TB;3` [font="Times] }}
qr7 X-[& >Iu]T{QNO u4`mQ6 [font="Times]Map举例
+R3\cRM [font="Times]import java.util.Map;
3(cU) [font="Times]import java.util.HashMap;
A%.J%[MVz [font="Times]import java.util.Iterator;
Q:'qw#P/C [font="Times]import java.io.FileReader;
]Y?{$M
G [font="Times]
bS_y_9K [font="Times]public class MapExample {
uEc0/a :. [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
cfrvy^>, [font="Times] Map word_count_map = new HashMap();
~| 4U@ [font="Times] FileReader reader = new FileReader(args[0]);
p} t{8j> [font="Times] Iterator words = new WordStreamIterator(reader);
V=G b>_d [font="Times]
pil0,r
$D [font="Times] while ( words.hasNext() ) {
x&Q+|b% [font="Times] String word = (String) words.next();
Z[DetRc- [font="Times] String word_lowercase = word.toLowerCase();
rC* sNy2 [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
rTWh(8T [font="Times]if ( frequency == null ) {
.rt8]% [font="Times] frequency = new Integer(1);
!:]s M-cCt [font="Times] } else {
>!:$@!6L [font="Times] int value = frequency.intValue();
#i}# jMT [font="Times] frequency = new Integer(value + 1);}
/k4^& [font="Times] word_count_map.put(word_lowercase, frequency);
OpWC2t) [font="Times] }
.E?bH V [font="Times] System.out.println(word_count_map);
chvrHvByS [font="Times] }}
4*@G&v?n .(TQ5/
~ uW\@x4 [font="Times]Queue举例:
GoGohsj [font="Times]import java.io.IOException;
<M5{.`o [font="Times]import java.io.PrintStream;
jsZiARTZRl [font="Times]import java.util.LinkedList;
/Bg6z m [font="Times]import java.util.Queue;
l(3'Re [font="Times]
se^NQ= [font="Times]public class QueueTester {
s$SU
vo1J [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
XvfcPI6 [font="Times] public QueueTester() {q = new LinkedList();}
7eaA]y~H [font="Times]public void testFIFO(PrintStream out) throws IOException {
yDu
yMt# [font="Times] q.add("First");
>
{'5>6u [font="Times] q.add("Second");
j?d;xj [font="Times] q.add("Third");
-D&.)N9ctQ [font="Times] Object o;
CS^ oiV%{s [font="Times] while ((o = q.poll()) != null) {
1B9Fb.i [font="Times] out.println(o);}}
Q2_WH)J 3 [font="Times] public static void main(String[] args) {
mG}^'?^K [font="Times] QueueTester tester = new QueueTester();
kuKnJWv [font="Times] try { tester.testFIFO(System.out);
GY0XWUlC [font="Times] } catch (IOException e) {
}5TfQV6 [font="Times] e.printStackTrace(); } }}
1)P<cNj CYTuj>Ww !:g>CDA 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
Y:tW] 总结:
Allt]P> 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
MHpL$g=5_ 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。